Cervical facet capsular ligament yield defines the threshold for injury and persistent joint-mediated neck pain

Mechanical properties of animal ligaments: a review and comparative study for the identification of the most suitable human ligament surrogates

The interest in the properties of animal soft tissues is often related to the desire to find an animal model to replace human counterparts due to the unsteady availability of human tissues for experimental purposes. Once the most appropriate animal model is identified, it is possible to carry out ex-vivo and in-vivo studies for the repair of ligamentous tissues and performance testing of replacement and support healing devices. This work aims to present a systematic review of the mechanical properties of ligaments reported in the scientific literature by considering different anatomical regions in humans and several animal species. This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method. Moreover, considering the lack of a standard protocol for preconditioning of tissues, this aspect is also addressed. Ninety-six studies were selected for the systematic review and analysed. The mechanical properties of different animal species are reported and summarised in tables. Only results from studies reporting the strain rate parameter were considered for comparison with human ligaments, as they were deemed more reliable. Elastic modulus, ultimate tensile stress, and ultimate strain properties are graphically reported identifying the range of values for each animal species and to facilitate comparison between values reported in the scientific literature in animal and human ligaments. Useful similarities between the mechanical properties of swine, cow, and rat and human ligaments have been found.

Intra-articular MMP-1 in the spinal facet joint induces sustained pain and neuronal dysregulation in the DRG and spinal cord, and alters ligament kinematics under tensile loading

Chronic joint pain is a major healthcare challenge with a staggering socioeconomic burden. Pain from synovial joints is mediated by the innervated collagenous capsular ligament that surrounds the joint and encodes nociceptive signals. The interstitial collagenase MMP-1 is elevated in painful joint pathologies and has many roles in collagen regulation and signal transduction. Yet, the role of MMP-1 in mediating nociception in painful joints remains poorly understood. The goal of this study was to determine whether exogenous intra-articular MMP-1 induces pain in the spinal facet joint and to investigate effects of MMP-1 on mediating the capsular ligament’s collagen network, biomechanical response, and neuronal regulation. Intra-articular MMP-1 was administered into the cervical C6/C7 facet joints of rats. Mechanical hyperalgesia quantified behavioral sensitivity before, and for 28 days after, injection. On day 28, joint tissue structure was assessed using histology. Multiscale ligament kinematics were defined under tensile loading along with microstructural changes in the collagen network. The amount of degraded collagen in ligaments was quantified and substance P expression assayed in neural tissue since it is a regulatory of nociceptive signaling. Intra-articular MMP-1 induces behavioral sensitivity that is sustained for 28 days (p < 0.01), absent any significant effects on the structure of joint tissues. Yet, there are changes in the ligament’s biomechanical and microstructural behavior under load. Ligaments from joints injected with MMP-1 exhibit greater displacement at yield (p = 0.04) and a step-like increase in the number of anomalous reorganization events of the collagen fibers during loading (p ≤ 0.02). Collagen hybridizing peptide, a metric of damaged collagen, is positively correlated with the spread of collagen fibers in the unloaded state after MMP-1 (p = 0.01) and that correlation is maintained throughout the sub-failure regime (p ≤ 0.03). MMP-1 injection increases substance P expression in dorsal root ganglia (p < 0.01) and spinal cord (p < 0.01) neurons. These findings suggest that MMP-1 is a likely mediator of neuronal signaling in joint pain and that MMP-1 presence in the joint space may predispose the capsular ligament to altered responses to loading. MMP-1-mediated pathways may be relevant targets for treating degenerative joint pain in cases with subtle or no evidence of structural degeneration.

The Use of Wearable Sensors for Preventing, Assessing, and Informing Recovery from Sport-Related Musculoskeletal Injuries: A Systematic Scoping Review

Wearable technologies are often indicated as tools that can enable the in-field collection of quantitative biomechanical data, unobtrusively, for extended periods of time, and with few spatial limitations. Despite many claims about their potential for impact in the area of injury prevention and management, there seems to be little attention to grounding this potential in biomechanical research linking quantities from wearables to musculoskeletal injuries, and to assessing the readiness of these biomechanical approaches for being implemented in real practice. We performed a systematic scoping review to characterise and critically analyse the state of the art of research using wearable technologies to study musculoskeletal injuries in sport from a biomechanical perspective. A total of 4952 articles were retrieved from the Web of Science, Scopus, and PubMed databases; 165 were included. Multiple study features-such as research design, scope, experimental settings, and applied context-were summarised and assessed. We also proposed an injury-research readiness classification tool to gauge the maturity of biomechanical approaches using wearables. Five main conclusions emerged from this review, which we used as a springboard to propose guidelines and good practices for future research and dissemination in the field.

Analysis of Functional Outcomes and Risk Factors for Facet Joint Distraction During Anterior Cervical Discectomy and Fusion for Cervical Spondylotic Myelopathy

OBJECTIVE

This study aimed to clarify functional outcomes of facet joint distraction (FJD) and identify specific risk factors for excessive FJD during single-level anterior cervical discectomy and fusion (ACDF) for cervical spondylotic myelopathy (CSM).

METHODS

This study retrospectively analyzed 100 patients who underwent single-level ACDF for CSM from January 2016 to May 2020. Anteroposterior and lateral radiographs were obtained before surgery and 12 months after surgery. Radiographic parameters including anterior intervertebral height (AIH), posterior intervertebral height, facet joint gap, cage posterior depth (CPD), upper vertebral length, cervical segmental Cobb angle (CSCA), C2-C7 Cobb angle, and C2-C7 sagittal vertical axis were analyzed. Functional outcomes were evaluated using the modified Japanese Orthopedic Association Score, visual analog scale (VAS), and Neck Disability Index (NDI).

RESULTS

Comparison between the appropriate FJD and excessive FJD groups showed statistically significant differences in the NDI, VAS, CPD, and ΔAIH (P < 0.05). Multivariate logistic regression analysis showed that independent factors associated with excessive FJD were as follows: a ΔAIH > 2.28 mm (odds ratio [OR] = 6.792, 95% confidence interval [CI] = 1.885-24.470, P = 0.003), CPD > 12.45 mm (OR = 5.876, 95% CI = 1.828-18.895, P = 0.003), and post-CSCA < 0° (OR = 6.251, 95% CI = 1.275-30.633, P = 0.024). Furthermore, receiver operating characteristic curve analysis for the multilevel logistic regression model produced an area under the curve of 0.783 (P < 0.001).

CONCLUSION

Patients with an FJD of >0.905 mm had worse NDI and VAS pain scores, but not a poorer modified Japanese Orthopedic Association Score recovery rate. Our findings suggested that a ΔAIH > 2.28 mm, CPD > 12.45 mm, and post-CSCA < 0° were independent risk factors for excessive FJD after single-level ACDF for CSM.

Characterizing the Mechanical and Viscoelastic Response of the Porcine Facet Joint Capsule Ligament in Response to a Simulated Impact

The facet capsule ligament (FCL) is a structure in the lumbar spine that constrains motions of the vertebrae. Subfailure loads can produce micro-damage resulting in increased laxity, decreased stiffness, and altered viscoelastic responses. Therefore, the purpose of this investigation was to determine the mechanical and viscoelastic properties of the FCL under various magnitudes of strain from control samples and samples that had been through an impact protocol. Two hundred FCL tissue samples were tested (20 Control & 180 Impacted). Impacted FCL tissue samples were obtained from functional spinal units that had been exposed to one of nine sub-failure impact conditions. All specimens underwent to following loading protocol: preconditioning with 5 cycles of 5% strain, followed by a 30 second rest period, 5 cycles of 10% strain and 1 cycle of 10% strain with a hold duration at 10% strain for 240 seconds (4 minutes). The same protocol followed for 30% and 50% strain. Measures of stiffness, hysteresis and force-relaxation were computed. No significant differences in stiffness were observed for impacted specimens in comparison to control. Impacted specimens from the 8g Flexed and 11g Flexed and Neutral conditions exhibited greater hysteresis during the cyclic-30% and cyclic-50% portion of the protocol in comparison to controls. In addition, specimens from the 8g and 11g Flexed conditions resulted in greater stress decay for the 50%-hold conditions. Results from this study demonstrate viscoelastic changes in FCL samples exposed to moderate and highspeed single impacts in a flexed posture.

Stretch-Induced Intimal Failure in Isolated Cerebral Arteries as a Function of Development

Recent clinical studies have shown that traumatic brain injury is a significant risk factor for stroke. Motivated to better understand possible mechanisms of this association, we studied subfailure disruption of the intima in overstretched sheep cerebral arteries, as this has been implicated in the increased risk of stroke following blunt cerebrovascular injury. Middle cerebral arteries from four age groups (ranging from fetal to adult) were stretched axially to failure, and intimal disruption was captured with a video camera. All vessels demonstrated intimal disruption prior to catastrophic failure, with nearly all incurring disruption at stretch values well below those at ultimate stress (means of 1.56 and 1.73, respectively); the lowest stretch associated with intimal disruption was 1.29. The threshold of intimal failure was independent of age. Additional analysis showed that disruption included failure of both the endothelium and internal elastic lamina. Although our experiments were conducted at quasi-static rates, the results likely have important implications for vessel function following trauma. Future work should seek to identify subfailure disruption of the cerebrovasculature in head trauma.

Sex, Age and Stature Affects Neck Biomechanical Responses in Frontal and Rear Impacts Assessed Using Finite Element Head and Neck Models

The increased incidence of injury demonstrated in epidemiological data for the elderly population, and females compared to males, has not been fully understood in the context of the biomechanical response to impact. A contributing factor to these differences in injury risk could be the variation in geometry between young and aged persons and between males and females. In this study, a new methodology, coupling a CAD and a repositioning software, was developed to reposture an existing Finite element neck while retaining a high level of mesh quality. A 5th percentile female aged neck model (F05_75YO) and a 50th percentile male aged neck model (M50_75YO) were developed from existing young (F05_26YO and M50_26YO) neck models (Global Human Body Models Consortium v5.1). The aged neck models included an increased cervical lordosis and an increase in the facet joint angles, as reported in the literature. The young and the aged models were simulated in frontal (2, 8, and 15 g) and rear (3, 7, and 10 g) impacts. The responses were compared using head and relative facet joint kinematics, and nominal intervertebral disc shear strain. In general, the aged models predicted higher tissue deformations, although the head kinematics were similar for all models. In the frontal impact, only the M50_75YO model predicted hard tissue failure, attributed to the combined effect of the more anteriorly located head with age, when compared to the M50_26YO, and greater neck length relative to the female models. In the rear impacts, the F05_75YO model predicted higher relative facet joint shear compared to the F05_26YO, and higher relative facet joint rotation and nominal intervertebral disc strain compared to the M50_75YO. When comparing the male models, the relative facet joint kinematics predicted by the M50_26YO and M50_75YO were similar. The contrast in response between the male and female models in the rear impacts was attributed to the higher lordosis and facet angle in females compared to males. Epidemiological data reported that females were more likely to sustain Whiplash Associated Disorders in rear impacts compared to males, and that injury risk increases with age, in agreement with the findings in the present study. This study demonstrated that, although the increased lordosis and facet angle did not affect the head kinematics, changes at the tissue level were considerable (e.g., 26% higher relative facet shear in the female neck compared to the male, for rear impact) and relatable to the epidemiology. Future work will investigate tissue damage and failure through the incorporation of aged material properties and muscle activation.

Importance of the cervical capsular joint cartilage geometry on head and facet joint kinematics assessed in a Finite element neck model

Finite element human neck models (NMs) aim to predict neck response and injury at the tissue level; however, contemporary models are most often assessed using global response such as head kinematics. Additionally, many NMs are developed from subject-specific imaging with limited soft tissue resolution in small structures such as the facet joints in the neck. Such details may be critical to enable NMs to predict tissue-level response. In the present study, the capsular joint cartilage (CJC) geometry in a contemporary NM was enhanced (M50-CJC) based on literature data. The M50-CJC was validated at the segment and full neck levels and assessed using relative facet joint kinematics (FJK), capsular ligament (CL) and intervertebral disc (IVD) strains, a relative vertebral rotation assessment (IV-NIC) and head kinematics in frontal and rear impact. The validation ratings at the segment level increased from 0.60 to 0.64, with improvements for modes of deformation associated with the facet joints, while no difference was noted at the head kinematic level. The improved CJC led to increased FJK rotation (188%) and IVD strain (152.2%,) attributed to the reduced facet joint gap. Further enhancements of the capsular joint representation or a link between the FJK and CL injury risk are recommended. Enhancements at the tissue level demonstrated a large effect on the IVD strain, but were not apparent in global metrics such as head kinematics. This study demonstrated that a biofidelic and detailed geometrical representation of the CJC contributes significantly to the predicted joint response, which is critical to investigate neck injury risk at the tissue level.

Equine Cervical Pain and Dysfunction: Pathology, Diagnosis and Treatment

Simple Summary

Neck pain and dysfunction in the horse is becoming an increasingly important topic among riders, trainers and veterinarians. Some horses may present for a subtle performance decline, while others may show dramatic, dangerous behavior. It is important to recognize how to carefully evaluate the horse in an effort to understand the different types of pain that may be contributing to the different behaviors. The musculoskeletal and nervous systems may both play a role in the development of clinical signs. Recognizing that there are many diagnostic options as well as several treatments choices is important. This synopsis covers the disease processes that may contribute to the development of neck pain and dysfunction in the horse, as well as several possible diagnostic and treatment options.

Abstract

Interest in the cervical spine as a cause of pain or dysfunction is increasingly becoming the focus of many equine practitioners. Many affected horses are presented for poor performance, while others will present with dramatic, sometimes dangerous behavior. Understanding and distinguishing the different types of neck pain is a starting point to comprehending how the clinical presentations can vary so greatly. There are many steps needed to systematically evaluate the various tissues of the cervical spine to determine which components are contributing to cervical pain and dysfunction. Osseous structures, soft tissues and the central and the peripheral nervous system may all play a role in these various clinical presentations. After completing the clinical evaluation, several imaging modalities may be implemented to help determine the underlying pathologic processes. There are multiple treatment options available and each must be carefully chosen for an individual horse. Provided is a synopsis of the current knowledge as to different disease processes that can result in cervical pain and dysfunction, diagnostic approaches and treatment strategies. Improving the knowledge in these areas will ideally help to return horses to a state of well-being that can be maintained over time and through the rigors of their job or athletic endeavors.

The rate of tendon failure in a collagen fibre recruitment-based model

Accurate characterization of the mechanical response of collagenous tissues is critical for investigations into mechanisms of soft tissue injury. These tissues are inherently viscoelastic, exhibiting strain-rate dependent stiffnesses, creep, and stress-relaxation. The strain-rate features of the failure portion of the stress-strain curve are less well developed. Collagen-distribution based models are improving and capable of reproducing the non-linear aspects of the elastic response of soft tissues, but still require parameterization of failure regions. Therefore, the purpose of this investigation, was to determine whether the parameters characterizing the rate of damage accumulation in a collagen-distribution model are proportional to strain rate. Fifty rat tail tendons were subjected to one of five different strain rates (0.01, 0.05, 0.1, 0.15, 0.20 s^-1) until failure in an uni-axial strain test. To test the hypothesis that the parameters associated with damage rate are proportional to strain rate, a collagen distribution model was employed with the parameters describing the rate of fibre damage being obtained by least-squares and regressed against the strain rate. The breaking function was found to be proportional to strain rate, with a proportionality constant of 60.7 s^-1. Properties characterizing the failure portion of the stress-strain curves for rat tail tendons are also reported. The Young's Modulus did not vary with strain rate and was found to be 103.3 ± 49.5 MPa. Similarly, failure stresses and strains did not vary across the strain rates tested, and were 15.6 ± 6.1 MPa and 32.2 ± 9.1%, respectively.

Biomechanical properties of canine staphylectomies closed with barbed or smooth suture

OBJECTIVE

To compare the duration of closure and biomechanical properties of staphylectomies closed with absorbable bidirectional barbed suture or smooth monofilament suture in a simple continuous or interrupted pattern STUDY DESIGN: Ex vivo study SAMPLE POPULATION: Soft palates (n = 60) harvested from mesaticephalic canine cadavers METHODS: One centimeter of tissue was excised from the caudal border of each soft palate, and the oral and nasopharyngeal mucosal surfaces were apposed with 2-0 bidirectional Quill Monoderm knotless closure device barbed suture (Q), 3-0 Monocryl in a simple continuous (MC) pattern, or 3-0 Monocryl in a simple interrupted (MI) pattern (n = 20 per group). Duration of closure was compared between groups. Tissues were tested under tension to failure, and mode of failure data were collected by video capture.

RESULTS

Closure time was longer for MI closures than for Q and MC closures, with means of 259.9, 215.4, and 196.7 seconds, respectively (P < .0001). No difference was detected in yield force, force to first tissue rupture, maximum force, and energy required for yield and maximum force between groups. Energy to yield was 190.0, 167.8, and 188.95 N-mm for MI, Q, and MC closures, respectively.

CONCLUSION

Biomechanical properties of staphylectomies closed with barbed or smooth sutures did not differ in this cadaveric model.

CLINICAL SIGNIFICANCE

Barbed suture can be considered as an alternative for closure of canine staphylectomies. These results provide evidence to justify additional research to evaluate clinical outcomes in dogs undergoing staphylectomy.

Material properties of human patellar-ligament grafts from the elderly population

The aim of the presented study was to estimate the material properties of human patellar ligaments from the elderly population by means of tensile tests. The experimental part was conducted on a custom tensile-testing device, with a built-in enclosure to simulate in-vivo conditions, using 25 (15 female, 10 male) bone-ligament-bone samples from elderly (age 83 (8)) human donors. During the tensile tests, the resultant force and displacement of the sample attachments were recorded. With this data and the values of the initial length and the initial cross-sectional area of the samples, the engineering stress and strain, the Young's modulus and the toughness at rupture were calculated for each sample. The results were then averaged and presented for all the samples together and for the female and male populations separately. The measured Young's modulus and the failure stress values were found to be significantly higher for the female samples compared to the male (p < 0.05). All the other measured properties did not show a significant difference. The toe region's material properties for the patellar ligament were also presented as valuable information for the anterior cruciate ligament reconstruction. The tensile-test results were compared to other research carried on human patellar ligaments using samples from younger donors. The comparison showed that the samples from the elderly population exhibit lower values of strain at the end of the toe region and have a lower failure strain for the patellar ligament. The Young's modulus and the failure stress of the samples in this study were in the range of other research conducted on patellar ligaments.

Analysis of the mechanical response of damaged human cervical spine ligaments

BACKGROUND

Cervical spine ligaments that protect the spinal cord and stabilize the spine are frequently injured in motor vehicle collisions and other traumatic situations. These injuries are usually incomplete, and often difficult to notice. The focus of the presented study is placed on analysis of the effect of subfailure load on the mechanical response of the three main cervical spine ligaments: the anterior and the posterior longitudinal ligament and the ligamentum flavum.

METHODS

A total of 115 samples of human cadaveric ligaments removed within 24-48 h after death have been tested. Uniaxial tension tests along the fiber direction were performed in physiological conditions on a custom designed test equipment. The ligaments were loaded into an expected damage zone at two different subfailure values (based on previously reported reference group of 46 samples), and then reloaded to failure.

FINDINGS

The main effect of a high subfailure load has proven to be the toe elongation change. The toe elongation increase is affected by the subfailure load value. While anterior and posterior longitudinal ligament showed similar changes, the smallest subfailure effect was found in ligamentum flavum.

INTERPRETATIONS

The normal physiological region of the cervical spine ligaments mechanical response is modified by a high subfailure load. The observed ligament injury significantly compromises ligament ability to give tensile support within physiological spinal motion.

Thresholds for the assessment of inflicted head injury by shaking trauma in infants: a systematic review

In order to investigate potential causal relations between the shaking of infants and injuries, biomechanical studies compare brain and skull dynamic behavior during shaking to injury thresholds. However, performing shaking tolerance research on infants, either in vivo or ex vivo, is extremely difficult, if not impossible. Therefore, infant injury thresholds are usually estimated by scaling or extrapolating adult or animal data obtained from crash tests or whiplash experiments. However, it is doubtful whether such data accurately matches the biomechanics of shaking in an infant. Hence some thresholds may be inappropriate to be used for the assessment of inflicted head injury by shaking trauma in infants. A systematic literature review was conducted to 1) provide an overview of existing thresholds for head- and neck injuries related to violent shaking, and 2) to identify and discuss which thresholds have been used or could be used for the assessment of inflicted head injury by shaking trauma in infants. Key findings: The majority of studies establishing or proposing injury thresholds were found to be based on loading cycle durations and loading cycle repetitions that did not resemble those occurring during shaking, or had experimental conditions that were insufficiently documented in order to evaluate the applicability of such thresholds. Injury thresholds that were applied in studies aimed at assessing whether an injury could occur under certain shaking conditions were all based on experiments that did not properly replicate the loading characteristics of shaking. Somewhat validated threshold scaling methods only exist for scaling concussive injury thresholds from adult primate to adult human. Scaling methods that have been used for scaling other injuries, or for scaling adult injury thresholds to infants were not validated. There is a clear and urgent need for new injury thresholds established by accurately replicating the loading characteristics of shaking.

Physiologic facet capsule stretch can induce pain & upregulate matrix metalloproteinase-3 in the dorsal root ganglia when preceded by a physiological mechanical or nonpainful chemical exposure

BACKGROUND

Neck pain from cervical facet loading is common and induces inflammation and upregulation of nerve growth factor (NGF) that can sensitize the joint afferents. Yet, the mechanisms by which these occur and whether afferents can be pre-conditioned by certain nonpainful stimuli are unknown. This study tested the hypothesis that a nonpainful mechanical or chemical insult predisposes a facet joint to generate pain after a later exposure to typically nonpainful distraction.

METHODS

Rats were exposed to either a nonpainful distraction or an intra-articular subthreshold dose of NGF followed by a nonpainful distraction two days later. Mechanical hyperalgesia was measured daily and C6 dorsal root ganglia (DRG) tissue was assayed for NGF and matrix metalloproteinase-3 (MMP-3) expression on day 7.

FINDINGS

The second distraction increased joint displacement and strains compared to its first application (p = 0.0011). None of the initial exposures altered behavioral sensitivity in either of the groups being pre-conditioned or in controls; but, sensitivity was established in both groups receiving a second distraction within one day that lasted until day 7 (p < 0.024). NGF expression in the DRG was increased in both groups undergoing a pre-conditioning exposure (p < 0.0232). Similar findings were observed for MMP-3 expression, with a pre-conditioning exposure increasing levels after an otherwise nonpainful facet distraction.

INTERPRETATION

These findings suggest that nonpainful insults to the facet joint, when combined, can generate painful outcomes, possibly mediated by upregulation of MMP-3 and mature NGF.

Come on baby, let's do the twist: the kinematics of killing in loggerhead shrikes

Shrikes use their beaks for procuring, dispatching and processing their arthropod and vertebrate prey. However, it is not clear how the raptor-like bill of this predatory songbird functions to kill vertebrate prey that may weigh more than the shrike itself. In this paper, using high-speed videography, we observed that upon seizing prey with their beaks, shrikes performed rapid (6-17 Hz; 49-71 rad s^-1) axial head-rolling movements. These movements accelerated the bodies of their prey about their own necks at g-forces of approximately 6 g, and may be sufficient to cause pathological damage to the cervical vertebrae and spinal cord. Thus, when tackling relatively large vertebrates, shrikes appear to use inertia of their prey's own body against them.

Repeated High Rate Facet Capsular Stretch at Strains That are Below the Pain Threshold Induces Pain and Spinal Inflammation With Decreased Ligament Strength in the Rat

Repeated loading of ligamentous tissues during repetitive occupational and physical tasks even within physiological ranges of motion has been implicated in the development of pain and joint instability. The pathophysiological mechanisms of pain after repetitive joint loading are not understood. Within the cervical spine, excessive stretch of the facet joint and its capsular ligament has been implicated in the development of pain. Although a single facet joint distraction (FJD) at magnitudes simulating physiologic strains is insufficient to induce pain, it is unknown whether repeated stretching of the facet joint and ligament may produce pain. This study evaluated if repeated loading of the facet at physiologic nonpainful strains alters the capsular ligament's mechanical response and induces pain. Male rats underwent either two subthreshold facet joint distractions (STFJDs) or sham surgeries each separated by 2 days. Pain was measured before the procedure and for 7 days; capsular mechanics were measured during each distraction and under tension at tissue failure. Spinal glial activation was also assessed to probe potential pathophysiologic mechanisms responsible for pain. Capsular displacement significantly increased (p = 0.019) and capsular stiffness decreased (p = 0.008) during the second distraction compared to the first. Pain was also induced after the second distraction and was sustained at day 7 (p < 0.048). Repeated loading weakened the capsular ligament with lower vertebral displacement (p = 0.041) and peak force (p = 0.014) at tissue rupture. Spinal glial activation was also induced after repeated loading. Together, these mechanical, physiological, and neurological findings demonstrate that repeated loading of the facet joint even within physiologic ranges of motion can be sufficient to induce pain, spinal inflammation, and alter capsular mechanics similar to a more injurious loading exposure.

Detection and characterization of molecular-level collagen damage in overstretched cerebral arteries

It is well established that overstretch of arteries alters their mechanics and compromises their function. However, the underlying structural mechanisms behind these changes are poorly understood. Utilizing a recently developed collagen hybridizing peptide (CHP), we demonstrate that a single mechanical overstretch of an artery produces molecular-level unfolding of collagen. In addition, imaging and quantification of CHP binding revealed that overstretch produces damage (unfolding) among fibers aligned with the direction of loading, that damage increases with overstretch severity, and that the onset of this damage is closely associated with tissue yielding. These findings held true for both axial and circumferential loading directions. Our results are the first to identify stretch-induced molecular damage to collagen in blood vessels. Furthermore, our approach is advantageous over existing methods of collagen damage detection as it is non-destructive, readily visualized, and objectively quantified. This work opens the door to revealing additional structure-function relationships in arteries. We anticipate that this approach can be used to better understand arterial damage in clinically relevant settings such as angioplasty and vascular trauma. Furthermore, CHP can be a tool for the development of microstructurally-based constitutive models and experimentally validated computational models of arterial damage and damage propagation across physical scales.

STATEMENT OF SIGNIFICANCE

Arteries play a critical role by carrying oxygen and essential nutrients throughout the body. However, trauma to the head and neck, as well as surgical interventions, can overstretch arteries and alter their mechanics. In order to better understand the cause of these changes, we employ a novel collagen hybridizing peptide (CHP) to study collagen damage in overstretched arteries. Our approach is unique in that we go beyond the fiber- and fibril-level and characterize molecular-level disruption. In addition, we image and quantify fluorescently-labeled CHP to reveal a new structure-property relationship in arterial damage. We anticipate that our approach can be used to better understand arterial damage in clinically relevant settings such as angioplasty and vascular trauma.

The Interface of Mechanics and Nociception in Joint Pathophysiology: Insights From the Facet and Temporomandibular Joints

Chronic joint pain is a widespread problem that frequently occurs with aging and trauma. Pain occurs most often in synovial joints, the body's load bearing joints. The mechanical and molecular mechanisms contributing to synovial joint pain are reviewed using two examples, the cervical spinal facet joints and the temporomandibular joint (TMJ). Although much work has focused on the macroscale mechanics of joints in health and disease, the combined influence of tissue mechanics, molecular processes, and nociception in joint pain has only recently become a focus. Trauma and repeated loading can induce structural and biochemical changes in joints, altering their microenvironment and modifying the biomechanics of their constitutive tissues, which themselves are innervated. Peripheral pain sensors can become activated in response to changes in the joint microenvironment and relay pain signals to the spinal cord and brain where pain is processed and perceived. In some cases, pain circuitry is permanently changed, which may be a potential mechanism for sustained joint pain. However, it is most likely that alterations in both the joint microenvironment and the central nervous system (CNS) contribute to chronic pain. As such, the challenge of treating joint pain and degeneration is temporally and spatially complicated. This review summarizes anatomy, physiology, and pathophysiology of these joints and the sensory pain relays. Pain pathways are postulated to be sensitized by many factors, including degeneration and biochemical priming, with effects on thresholds for mechanical injury and/or dysfunction. Initiators of joint pain are discussed in the context of clinical challenges including the diagnosis and treatment of pain.

The Physiological Basis of Cervical Facet-Mediated Persistent Pain: Basic Science and Clinical Challenges

Synopsis Chronic neck pain is a common condition and a primary clinical symptom of whiplash and other spinal injuries. Loading-induced neck injuries produce abnormal kinematics between the vertebrae, with the potential to injure facet joints and the afferent fibers that innervate the specific joint tissues, including the capsular ligament. Mechanoreceptive and nociceptive afferents that innervate the facet have their peripheral terminals in the capsule, cell bodies in the dorsal root ganglia, and terminal processes in the spinal cord. As such, biomechanical loading of these afferents can initiate nociceptive signaling in the peripheral and central nervous systems. Their activation depends on the local mechanical environment of the joint and encodes the neural processes that initiate pain and lead to its persistence. This commentary reviews the complex anatomical, biomechanical, and physiological consequences of facet-mediated whiplash injury and pain. The clinical presentation of facet-mediated pain is complex in its sensory and emotional components. Yet, human studies are limited in their ability to elucidate the physiological mechanisms by which abnormal facet loading leads to pain. Over the past decade, however, in vivo models of cervical facet injury that reproduce clinical pain symptoms have been developed and used to define the complicated and multifaceted electrophysiological, inflammatory, and nociceptive signaling cascades that are involved in the pathophysiology of whiplash facet pain. Integrating the whiplash-like mechanics in vivo and in vitro allows transmission of pathophysiological mechanisms across scales, with the hope of informing clinical management. Yet, despite these advances, many challenges remain. This commentary further describes and highlights such challenges. J Orthop Sports Phys Ther 2017;47(7):450-461. Epub 16 Jun 2017. doi:10.2519/jospt.2017.7255.

Evaluation of biofidelity of THUMS pedestrian model under a whole-body impact conditions with a generic sedan buck

OBJECTIVE

The goal of this study was to evaluate the biofidelity of the Total Human Model for Safety (THUMS; Ver. 4.01) pedestrian finite element models (PFEM) in a whole-body pedestrian impact condition using a well-characterized generic pedestrian buck model.

METHODS

The biofidelity of THUMS PFEM was evaluated with respect to data from 3 full-scale postmortem human subject (PMHS) pedestrian impact tests, in which a pedestrian buck laterally struck the subjects using a pedestrian buck at 40 km/h. The pedestrian model was scaled to match the anthropometry of the target subjects and then positioned to match the pre-impact postures of the target subjects based on the 3-dimensional motion tracking data obtained during the experiments. An objective rating method was employed to quantitatively evaluate the correlation between the responses of the models and the PMHS. Injuries in the models were predicted both probabilistically and deterministically using empirical injury risk functions and strain measures, respectively, and compared with those of the target PMHS.

RESULTS

In general, the model exhibited biofidelic kinematic responses (in the Y-Z plane) regarding trajectories (International Organization for Standardization [ISO] ratings: Y = 0.90 ± 0.11, Z = 0.89 ± 0.09), linear resultant velocities (ISO ratings: 0.83 ± 0.07), accelerations (ISO ratings: Y = 0.58 ± 0.11, Z = 0.52 ± 0.12), and angular velocities (ISO ratings: X = 0.48 ± 0.13) but exhibited stiffer leg responses and delayed head responses compared to those of the PMHS. This indicates potential biofidelity issues with the PFEM for regions below the knee and in the neck. The model also demonstrated comparable reaction forces at the buck front-end regions to those from the PMHS tests. The PFEM generally predicted the injuries that the PMHS sustained but overestimated injuries in the ankle and leg regions.

CONCLUSIONS

Based on the data considered, the THUMS PFEM was considered to be biofidelic for this pedestrian impact condition and vehicle. Given the capability of the model to reproduce biomechanical responses, it shows potential as a valuable tool for developing novel pedestrian safety systems.

The Role of the Trigemino Cervical Complex in Chronic Whiplash Associated Headache: A Cross Sectional Study

OBJECTIVE

To investigate signs of central sensitization in a cohort of patients with chronic whiplash associated headache (CWAH).

BACKGROUND

Central sensitization is one of the mechanisms leading to chronicity of primary headache, and thus might contribute to CWAH. However, the pathophysiological mechanism of CWAH is poorly understood and whether it is simply an expression of the primary headache or has a distinct pathogenesis remains unclear. Thus, the factors involved in the genesis of CWAH require further investigation.

METHODS

Twenty-two patients with CWAH (20 females, 2 males; age 25-50 years, mean age 36.3 years) and 25 asymptomatic participants (13 females, 12 males; age 18-50 years, mean age 35.6 years) rated glare and light-induced discomfort in response to light from an ophthalmoscope. Hyperalgesia evoked by a pressure algometer was assessed bilaterally on the forehead, temples, occipital base, and the middle phalanx of the third finger. The number, latency, area under the curve, and recovery cycle of nociceptive blink reflexes elicited by a supraorbital electrical stimulus were also recorded.

RESULTS

Eight and 6 CWAH patients had migrainous and tension-type headache (TTH) profiles, respectively; the remainder had features attributable to both migraine and TTH. Patients in the whiplash group reported significantly greater light-induced pain than controls (8.48 ± .35 vs 6.66 ± .43 on a 0-10 scale; P = .001). The CWAH patients reported significantly lower pressure pain thresholds at all sites. For stimuli delivered at 20 second intervals, whiplash patients were more responsive than controls (4.8 ± .6 blinks vs 3.0 ± .6 blinks in a block of 10 stimuli; P = .036). While R2 latencies and the area under the curve for the 20 second interval trials were comparable in both groups, there was a significant reduction of the area under the curve from the first to the second of the 2-second interval trials only in controls (99 ± 8% of baseline in whiplash patients vs 68 ± 7% in controls; P = .009). The recovery cycle was comparable for both groups.

CONCLUSIONS

Our results corroborate previous findings of mechanical hypersensitivity and photophobia in CWAH patients. The neurophysiological data provide further evidence for hyperexcitability in central nociceptive pathways, and endorse the hypothesis that CWAH may be driven by central sensitization.

Chronic whiplash pain

Although most patients recover from acute whiplash injuries, those with chronic whiplash syndrome develop signs of central nervous system (CNS) amplification of pain and have a poor prognosis. In this context, specific pain generators from acute whiplash have been identified through clinical, biomechanical, and animal studies. This article gives a clinical perspective on current understanding of these pain generators, including the phenomenon of CNS sensitization.

Development of the mechanical properties of engineered skin substitutes after grafting to full-thickness wounds

Engineered skin substitutes (ESSs) have been reported to close full-thickness burn wounds but are subject to loss from mechanical shear due to their deficiencies in tensile strength and elasticity. Hypothetically, if the mechanical properties of ESS matched those of native skin, losses due to shear or fracture could be reduced. To consider modifications of the composition of ESS to improve homology with native skin, biomechanical analyses of the current composition of ESS were performed. ESSs consist of a degradable biopolymer scaffold of type I collagen and chondroitin-sulfate (CGS) that is populated sequentially with cultured human dermal fibroblasts (hF) and epidermal keratinocytes (hK). In the current study, the hydrated biopolymer scaffold (CGS), the scaffold populated with hF dermal skin substitute (DSS), or the complete ESS were evaluated mechanically for linear stiffness (N/mm), ultimate tensile load at failure (N), maximum extension at failure (mm), and energy absorbed up to the point of failure (N-mm). These biomechanical end points were also used to evaluate ESS at six weeks after grafting to full-thickness skin wounds in athymic mice and compared to murine autograft or excised murine skin. The data showed statistically significant differences (p <0.05) between ESS in vitro and after grafting for all four structural properties. Grafted ESS differed statistically from murine autograft with respect to maximum extension at failure, and from intact murine skin with respect to linear stiffness and maximum extension. These results demonstrate rapid changes in mechanical properties of ESS after grafting that are comparable to murine autograft. These values provide instruction for improvement of the biomechanical properties of ESS in vitro that may reduce clinical morbidity from graft loss.

Gross anatomy of the deep perivertebral musculature in horses

OBJECTIVE

To determine the gross morphology of the multifidus, longus colli, and longus thoracis muscles in the cervical and cranial thoracic portions of the equine vertebral column.

SAMPLE

15 horse cadavers.

PROCEDURES

The vertebral column was removed intact from the first cervical vertebra (C1) to the seventh thoracic vertebra (T7). After removing the superficial musculature, detailed anatomic dissections of the multifidus, longus colli, and longus thoracis muscles were performed.

RESULTS

The multifidus cervicis muscle consisted of 5 bundles/level arranged in lateral, medial, and deep layers from C2 caudally into the thoracic portion of the vertebral column. Fibers in each bundle attached cranially to a spinous process then diverged laterally, attaching caudally on the dorsolateral edge of the vertebral lamina and blending into the joint capsule of an articular process articulation after crossing 1 to 4 intervertebral joints. The longus colli muscle had ventral, medial, and deep layers with 5 bundles/level from C1 to C5 that attached cranially to the ventral surface of the vertebral body, diverged laterally and crossed 1 to 4 intervertebral joints, then attached onto a vertebral transverse process as far caudally as C6. The longus thoracis muscle consisted of a single, well-defined muscle belly from C6 to T5-T6, with intermediate muscular attachments onto the ventral aspects of the vertebral bodies, the intervertebral symphyses, and the craniomedial aspects of the costovertebral joint capsules.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that there were multiple, short bundles of the multifidus cervicis, multifidus thoracis, and longus colli muscles; this was consistent with a function of providing sagittal plane intersegmental vertebral column stability.

Chronic neck pain: making the connection between capsular ligament laxity and cervical instability

The use of conventional modalities for chronic neck pain remains debatable, primarily because most treatments have had limited success. We conducted a review of the literature published up to December 2013 on the diagnostic and treatment modalities of disorders related to chronic neck pain and concluded that, despite providing temporary relief of symptoms, these treatments do not address the specific problems of healing and are not likely to offer long-term cures. The objectives of this narrative review are to provide an overview of chronic neck pain as it relates to cervical instability, to describe the anatomical features of the cervical spine and the impact of capsular ligament laxity, to discuss the disorders causing chronic neck pain and their current treatments, and lastly, to present prolotherapy as a viable treatment option that heals injured ligaments, restores stability to the spine, and resolves chronic neck pain. The capsular ligaments are the main stabilizing structures of the facet joints in the cervical spine and have been implicated as a major source of chronic neck pain. Chronic neck pain often reflects a state of instability in the cervical spine and is a symptom common to a number of conditions described herein, including disc herniation, cervical spondylosis, whiplash injury and whiplash associated disorder, postconcussion syndrome, vertebrobasilar insufficiency, and Barré-Liéou syndrome. When the capsular ligaments are injured, they become elongated and exhibit laxity, which causes excessive movement of the cervical vertebrae. In the upper cervical spine (C0-C2), this can cause a number of other symptoms including, but not limited to, nerve irritation and vertebrobasilar insufficiency with associated vertigo, tinnitus, dizziness, facial pain, arm pain, and migraine headaches. In the lower cervical spine (C3-C7), this can cause muscle spasms, crepitation, and/or paresthesia in addition to chronic neck pain. In either case, the presence of excessive motion between two adjacent cervical vertebrae and these associated symptoms is described as cervical instability. Therefore, we propose that in many cases of chronic neck pain, the cause may be underlying joint instability due to capsular ligament laxity. Currently, curative treatment options for this type of cervical instability are inconclusive and inadequate. Based on clinical studies and experience with patients who have visited our chronic pain clinic with complaints of chronic neck pain, we contend that prolotherapy offers a potentially curative treatment option for chronic neck pain related to capsular ligament laxity and underlying cervical instability.

Failure Properties and Damage of Cervical Spine Ligaments, Experiments and Modeling

Cervical spine ligaments have an important role in providing spinal cord stability and restricting excessive movements. Therefore, it is of great importance to study the mechanical properties and model the response of these ligaments. The aim of this study is to characterize the aging effects on the failure properties and model the damage of three cervical spine ligaments: the anterior and the posterior longitudinal ligament and the ligamentum flavum. A total of 46 samples of human cadaveric ligaments removed within 24–48 h after death have been tested. Uniaxial tension tests along the fiber direction were performed in physiological conditions. The results showed that aging decreased the failure properties of all three ligaments (failure load, failure elongation). Furthermore, the reported nonlinear response of cervical ligaments has been modeled with a combination of the previously reported hyperelastic and damage model. The model predicted a nonlinear response and damage region. The model fittings are in agreement with the experimental data and the quality of agreement is represented with the values of the coefficient of determination close to 1.

Engagement techniques and playing level impact the biomechanical demands on rugby forwards during machine-based scrummaging

OBJECTIVES

This cross-sectional study investigated the factors that may influence the physical loading on rugby forwards performing a scrum by studying the biomechanics of machine-based scrummaging under different engagement techniques and playing levels.

METHODS

34 forward packs from six playing levels performed repetitions of five different types of engagement techniques against an instrumented scrum machine under realistic training conditions. Applied forces and body movements were recorded in three orthogonal directions.

RESULTS

The modification of the engagement technique altered the load acting on players. These changes were in a similar direction and of similar magnitude irrespective of the playing level. Reducing the dynamics of the initial engagement through a fold-in procedure decreased the peak compression force, the peak downward force and the engagement speed in excess of 30%. For example, peak compression (horizontal) forces in the professional teams changed from 16.5 (baseline technique) to 8.6 kN (fold-in procedure). The fold-in technique also reduced the occurrence of combined high forces and head-trunk misalignment during the absorption of the impact, which was used as a measure of potential hazard, by more than 30%. Reducing the initial impact did not decrease the ability of the teams to produce sustained compression forces.

CONCLUSIONS

De-emphasising the initial impact against the scrum machine decreased the mechanical stresses acting on forward players and may benefit players' welfare by reducing the hazard factors that may induce chronic degeneration of the spine.

A modified prebind engagement process reduces biomechanical loading on front row players during scrummaging: a cross-sectional study of 11 elite teams

AIM

Biomechanical studies of the rugby union scrum have typically been conducted using instrumented scrum machines, but a large-scale biomechanical analysis of live contested scrummaging is lacking. We investigated whether the biomechanical loading experienced by professional front row players during the engagement phase of live contested rugby scrums could be reduced using a modified engagement procedure.

METHODS

Eleven professional teams (22 forward packs) performed repeated scrum trials for each of the three engagement techniques, outdoors, on natural turf. The engagement processes were the 2011/2012 (referee calls crouch-touch-pause-engage), 2012/2013 (referee calls crouch-touch-set) and 2013/2014 (props prebind with the opposition prior to the 'Set' command; PreBind) variants. Forces were estimated by pressure sensors on the shoulders of the front row players of one forward pack. Inertial Measurement Units were placed on an upper spine cervical landmark (C7) of the six front row players to record accelerations. Players' motion was captured by multiple video cameras from three viewing perspectives and analysed in transverse and sagittal planes of motion.

RESULTS

The PreBind technique reduced biomechanical loading in comparison with the other engagement techniques, with engagement speed, peak forces and peak accelerations of upper spine landmarks reduced by approximately 20%. There were no significant differences between techniques in terms of body kinematics and average force during the sustained push phase.

CONCLUSIONS

Using a scrum engagement process which involves binding with the opposition prior to the engagement reduces the stresses acting on players and therefore may represent a possible improvement for players' safety.

Injury and biomechanical perspectives on the rugby scrum: a review of the literature

As a collision sport, rugby union has a relatively high overall injury incidence, with most injuries being associated with contact events. Historically, the set scrum has been a focus of the sports medicine community due to the perceived risk of catastrophic spinal injury during scrummaging. The contemporary rugby union scrum is a highly dynamic activity but to this point has not been well characterised mechanically. In this review, we synthesise the available research literature relating to the medical and biomechanical aspects of the rugby union scrum, in order to (1) review the injury epidemiology of rugby scrummaging; (2) consider the evidence for specific injury mechanisms existing to cause serious scrum injuries and (3) synthesise the information available on the biomechanics of scrummaging, primarily with respect to force production. The review highlights that the incidence of acute injury associated with scrummaging is moderate but the risk per event is high. The review also suggests an emerging acknowledgement of the potential for scrummaging to lead to premature chronic degeneration injuries of the cervical spine and summarises the mechanisms by which these chronic injuries are thought to occur. More recent biomechanical studies of rugby scrummaging confirm that scrum engagement forces are high and multiplanar, but can be altered through modifications to the scrum engagement process which control the engagement velocity. As the set scrum is a relatively 'controlled' contact situation within rugby union, it remains an important area for intervention with a long-term goal of injury reduction.

Muscle pathologies after cervical spine distortion-like exposure--a porcine model

OBJECTIVE

Histological evaluation of porcine posterior cervical muscles after a forceful translational and extensional head retraction simulating high-speed rear end impact.

METHODS

Four anesthetized pigs were exposed to a cervical spine distortion (CSD)-like motion in a lying position. After 2 different survival times of 4 and 6 h (posttrauma), the pigs were euthanized and tissue sampling of posterior cervical muscles was performed. A standard histological staining method involving paraffin-embedded sections was used to analyze the muscles, focusing on injury signs like hemorrhage and inflammatory cell reaction. A pig that was not subjected to impact was used as a control pig and was subjected to the same procedure to exclude any potential artifacts from the autopsy.

RESULTS

The differentiation of 8 different posterior neck muscles in the dissection process was successful in more than 50 percent for each muscle of interest. Staining and valid analysis was possible from all extracted samples. Muscle injuries to the deepest posterior neck muscles could be found, especially in the musculus obliquus samples, which showed laminar bleedings in 4 out of 4 samples. In addition, in 4 out of 4 samples we were able to see increased cellular reactions. The splenius muscle also showed bleeding in all 4 samples. All animals showed muscle injury signs in more than three quarters of analyzed neck muscles. Differences between survival times of 4 and 6 h in terms of muscular injury were not of primary interest and could not be found.

CONCLUSIONS

By simulating a CSD-like motion we were able to confirm injuries in the posterior cervical muscles under severe loading conditions. Further studies need to be conducted to determine whether these muscle injuries also occur under lower exposure forces.

Mechanisms of chronic pain from whiplash injury

This article is to provide insights into the mechanisms underlying chronic pain from whiplash injury. Studies show that injury produces plasticity changes of different neuronal structures that are responsible for amplification of nociception and exaggerated pain responses. There is consistent evidence for hypersensitivity of the central nervous system to sensory stimulation in chronic pain after whiplash injury. Tissue damage, detected or not by the available diagnostic methods, is probably the main determinant of central hypersensitivity. Different mechanisms underlie and co-exist in the chronic whiplash condition. Spinal cord hyperexcitability in patients with chronic pain after whiplash injury can cause exaggerated pain following low intensity nociceptive or innocuous peripheral stimulation. Spinal hypersensitivity may explain pain in the absence of detectable tissue damage. Whiplash is a heterogeneous condition with some individuals showing features suggestive of neuropathic pain. A predominantly neuropathic pain component is related to a higher pain/disability level.

Maximum principal strain correlates with spinal cord tissue damage in contusion and dislocation injuries in the rat cervical spine

The heterogeneity of the primary mechanical mechanism of spinal cord injury (SCI) is not currently used to tailor treatment strategies because the effects of these distinct patterns of acute mechanical damage on long-term neuropathology have not been fully investigated. A computational model of SCI enables the dynamic analysis of mechanical forces and deformations within the spinal cord tissue that would otherwise not be visible from histological tissue sections. We created a dynamic, three-dimensional finite element (FE) model of the rat cervical spine and simulated contusion and dislocation SCI mechanisms. We investigated the relationship between maximum principal strain and tissue damage, and compared primary injury patterns between mechanisms. The model incorporated the spinal cord white and gray matter, the dura mater, cerebrospinal fluid, spinal ligaments, intervertebral discs, a rigid indenter and vertebrae, and failure criteria for ligaments and vertebral endplates. High-speed (∼ 1 m/sec) contusion and dislocation injuries were simulated between vertebral levels C3 and C6 to match previous animal experiments, and average peak maximum principal strains were calculated for several regions at the injury epicenter and at 1-mm intervals from +5 mm rostral to -5 mm caudal to the lesion. Average peak principal strains were compared to tissue damage measured previously in the same regions via axonal permeability to 10-kD fluorescein-dextran. Linear regression of tissue damage against peak maximum principal strain for pooled data within all white matter regions yielded similar and significant (p<0.0001) correlations for both contusion (R(2)=0.86) and dislocation (R(2)=0.52). The model enhances our understanding of the differences in injury patterns between SCI mechanisms, and provides further evidence for the link between principal strain and tissue damage.

Innervation of cervical ventral facet joint capsule: Histological evidence

AIM: To assess the presence of nerves in ventral facet joint capsules as facet capsules are generally implicated in neck pain.

METHODS: Twenty-four ventral cervical facet joint capsules were harvested from 3 unembalmed cadavers. Paraffin sections from these capsules were processed to identify neurofilament and substance P immunoreactive fibers. Nerve fiber presence was also verified by a silver impregnation method.

RESULTS: Neurofilament reactive fibers were observed in sections from 9 capsules. They were observed in areas with collagen fibers and areas with irregular connective tissue. Substance P reactive nerve fibers were found in sections from 7 capsules in similar areas. Silver impregnation also revealed the presence of nerve fibers. The nerve fibers were also found as bundles in the lateral margins of the capsule. A Pacinian corpuscle-like ending was also observed in one specimen.

CONCLUSION: Nerve fibers revealed by neurofilament immunoreactivity and silver staining support innervation of the ventral aspect of the facet joint capsule. The presence of substance P reactive fibers supports the potential role of these elements in mediating pain. The presence of a Pacinian-like ending implicates a potential role in joint movement.

The role of tissue damage in whiplash-associated disorders: discussion paper 1

STUDY DESIGN

Nonsystematic review of cervical spine lesions in whiplash-associated disorders (WAD).

OBJECTIVE

To describe whiplash injury models in terms of basic and clinical science, to summarize what can and cannot be explained by injury models, and to highlight future research areas to better understand the role of tissue damage in WAD.

SUMMARY OF BACKGROUND DATA

The frequent lack of detectable tissue damage has raised questions about whether tissue damage is necessary for WAD and what role it plays in the clinical context of WAD.

METHODS

Nonsystematic review.

RESULTS

Lesions of various tissues have been documented by numerous investigations conducted in animals, cadavers, healthy volunteers, and patients. Most lesions are undetected by imaging techniques. For zygapophysial (facet) joints, lesions have been predicted by bioengineering studies and validated through animal studies; for zygapophysial joint pain, a valid diagnostic test and a proven treatment are available. Lesions of dorsal root ganglia, discs, ligaments, muscles, and vertebral artery have been documented in biomechanical and autopsy studies, but no valid diagnostic test is available to assess their clinical relevance. The proportion of WAD patients in whom a persistent lesion is the major determinant of ongoing symptoms is unknown. Psychosocial factors, stress reactions, and generalized hyperalgesia have also been shown to predict WAD outcomes.

CONCLUSION

There is evidence supporting a lesion-based model in WAD. Lack of macroscopically identifiable tissue damage does not rule out the presence of painful lesions. The best available evidence concerns zygapophysial joint pain. The clinical relevance of other lesions needs to be addressed by future research.

On cervical zygapophysial joint pain after whiplash

STUDY DESIGN

Narrative review.

OBJECTIVE

To summarize the evidence that implicates the cervical zygapophysial joints as the leading source of chronic neck pain after whiplash.

SUMMARY OF BACKGROUND DATA

Reputedly a patho-anatomic basis for neck pain after whiplash has been elusive. However, studies conducted in a variety of disparate disciplines indicate that this is not necessarily the case.

METHODS

Data were retrieved from studies that addressed the postmortem features and biomechanics of injury to the cervical zygapophysial joints, and from clinical studies of the diagnosis and treatment of zygapophysial joint pain, to illustrate convergent validity.

RESULTS

Postmortem studies show that a spectrum of injuries can befall the zygapophysial joints in motor vehicle accidents. Biomechanics studies of normal volunteers and of cadavers reveal the mechanisms by which such injuries can be sustained. Studies in cadavers and in laboratory animals have produced these injuries.Clinical studies have shown that zygapophysial joint pain is very common among patients with chronic neck pain after whiplash, and that this pain can be successfully eliminated by radiofrequency neurotomy.

CONCLUSION

The fact that multiple lines of evidence, using independent techniques, consistently implicate the cervical zygapophysial joints as a site of injury and source of pain, strongly implicates injury to these joints as a common basis for chronic neck pain after whiplash.

Spinal facet joint biomechanics and mechanotransduction in normal, injury and degenerative conditions

The facet joint is a crucial anatomic region of the spine owing to its biomechanical role in facilitating articulation of the vertebrae of the spinal column. It is a diarthrodial joint with opposing articular cartilage surfaces that provide a low friction environment and a ligamentous capsule that encloses the joint space. Together with the disc, the bilateral facet joints transfer loads and guide and constrain motions in the spine due to their geometry and mechanical function. Although a great deal of research has focused on defining the biomechanics of the spine and the form and function of the disc, the facet joint has only recently become the focus of experimental, computational and clinical studies. This mechanical behavior ensures the normal health and function of the spine during physiologic loading but can also lead to its dysfunction when the tissues of the facet joint are altered either by injury, degeneration or as a result of surgical modification of the spine. The anatomical, biomechanical and physiological characteristics of the facet joints in the cervical and lumbar spines have become the focus of increased attention recently with the advent of surgical procedures of the spine, such as disc repair and replacement, which may impact facet responses. Accordingly, this review summarizes the relevant anatomy and biomechanics of the facet joint and the individual tissues that comprise it. In order to better understand the physiological implications of tissue loading in all conditions, a review of mechanotransduction pathways in the cartilage, ligament and bone is also presented ranging from the tissue-level scale to cellular modifications. With this context, experimental studies are summarized as they relate to the most common modifications that alter the biomechanics and health of the spine-injury and degeneration. In addition, many computational and finite element models have been developed that enable more-detailed and specific investigations of the facet joint and its tissues than are provided by experimental approaches and also that expand their utility for the field of biomechanics. These are also reviewed to provide a more complete summary of the current knowledge of facet joint mechanics. Overall, the goal of this review is to present a comprehensive review of the breadth and depth of knowledge regarding the mechanical and adaptive responses of the facet joint and its tissues across a variety of relevant size scales.

Pressure measurement in the cervical spinal facet joint: considerations for maintaining joint anatomy and an intact capsule

STUDY DESIGN

A novel noninvasive approach to measure facet joint pressure in the cervical spine was investigated using a tip-mounted transducer that can be inserted through a hole in the bony lateral mass. This technique is advantageous because it does not require resection of the joint capsule, but there are potential issues regarding its applicability that are addressed.

OBJECTIVE

The objective was to evaluate the effect of a tip-mounted pressure probe's position and orientation on contact pressure measurements in biomechanical experiments.

SUMMARY OF BACKGROUND DATA

Measurements of direct contact pressure in the facet joint of cadaveric spines have been obtained via pressure-sensitive films. However, that method requires the resection of the facet capsule, which can alter the overall joint's mechanical behavior and can affect the measured contact pressures.

METHODS

Influence of position and orientation on probe measurements was evaluated in companion surrogate and cadaveric investigations. The probe was placed in the facet of an anatomic vertebral C4/5 surrogate undergoing sagittal bending moments. Pressure-sensitive paper was used to map contact regions in the joint of the surrogate and cadaveric cervical segments (n = 3) during extension. The probe also underwent uniaxial compression in cadaveric facets to evaluate the effect of orientation relative to the contact surface on the probe signal.

RESULTS

Although experimental and theoretical pressure profiles followed the same trends, measured maximum pressures were half of the theoretical ones. In the orientation study, maximum pressures were not different for probe orientations of 0° and 5°, but no signal was recorded at orientations greater than 15°.

CONCLUSION

This approach to measure pressure was selected to provide a minimally-invasive method to quantify facet joint pressures during clinically relevant applications. Both the position and orientation of the probe are critical factors in monitoring local pressure profiles in this mobile synovial joint.

Incorporation of lower neck shear forces to predict facet joint injury risk in low-speed automotive rear impacts

Lower neck shear force remains a viable candidate for a low-velocity automotive rear-impact injury criterion. Data were previously reported to demonstrate high correlations between the magnitude of lower neck shear force and lower cervical spine facet joint motions. The present study determined the ability of lower neck shear force to predict soft-tissue injury risk in simulated automotive rear impacts. Rear-impact tests were conducted at two velocities and with two seatback orientations using a Hybrid III anthropomorphic test device (ATD) and stock automobile seats from 2007 model year vehicles. Higher velocities and more vertical seatback orientations were associated with higher injury risk based on computational modeling simulations performed in this study. Six cervical spine injury criteria including NIC, Nij, Nkm, LNL, and lower neck shear force and bending moment, increased with impact velocity. NIC, Nij, and shear force were most sensitive to changes in impact velocity. Four metrics, including Nkm, LNL, and lower neck shear force and bending moment, increased for tests with more vertical seatback orientations. Shear force was most sensitive to changes in seatback orientation. Peak values for shear force, NIC, and Nij occurred approximately at the time of head restraint contact for all four test conditions. Therefore, of the six investigated metrics, lower neck shear force was the only metric to demonstrate consistency with regard to injury risk and timing of peak magnitudes. These results demonstrate the ability of lower neck shear force to predict injury risk during low velocity automotive rear impacts and warrant continued investigation into the sensitivity and applicability of this metric for other rear-impact conditions.

Anomalous fiber realignment during tensile loading of the rat facet capsular ligament identifies mechanically induced damage and physiological dysfunction

Many pathophysiological phenomena are associated with soft tissue loading that does not produce visible damage or tissue failure. As such, there is an unexplained disconnect between tissue injury and detectable structural damage during loading. This study investigated the collagen fiber kinematics of the rat facet capsular ligament to identify the onset of subfailure damage during tensile loading conditions that are known to induce pain. Quantitative polarized light imaging was used to determine the collagen fiber orientation in the capsular ligament (n=7) under tension, and an alignment vector correlation measurement was employed to identify local anomalous fiber realignment during loading. During the initial portion of loading when tissue stiffness was increasing, anomalous realignment was more likely to be detected than mechanical evidence of structural damage, and as a result, anomalous fiber realignment was identified significantly (p=0.004) before gross failure. The occurrence of anomalous fiber realignment was significantly associated (p=0.013) with a decrease in tangent stiffness during loading (ligament yield), suggesting this optical metric may be associated with a loss of structural integrity. The presence of localized anomalous realignment during subfailure loading in this tissue may explain the development of laxity, collagen fiber disorganization, and persistent pain previously reported for facet joint distractions comparable to that required for anomalous realignment. These optical data, together with the literature, suggest that mechanically induced tissue damage may occur in the absence of any macroscopic or mechanical evidence of failure and may produce local pathology and pain.

Vector correlation technique for pixel-wise detection of collagen fiber realignment during injurious tensile loading

Excessive soft tissue loading can produce adverse structural and physiological changes in the absence of any visible tissue rupture. However, image-based analysis techniques to assess microstructural changes during loading without any visible rupture remain undeveloped. Quantitative polarized light imaging (QPLI) can generate spatial maps of collagen fiber alignment during loading with high temporal resolution and can provide a useful technique to measure microstructural responses. While collagen fibers normally realign in the direction that tissue is loaded, rapid, atypical fiber realignment during loading may be associated with the response of a local collagenous network to fiber failure. A vector correlation technique was developed to detect this atypical fiber realignment using QPLI and mechanical data collected from human facet capsular ligaments (n=16) loaded until visible rupture. Initial detection of anomalous realignment coincided with a measurable decrease in the tissue stiffness in every specimen and occurred at significantly lower strains than those at visible rupture (p<0.004), suggesting this technique may be sensitive to a loss of microstructural integrity. The spatial location of anomalous realignment was significantly associated with regions where visible rupture developed (p<0.001). This analysis technique provides a foundation to identify regional differences in soft tissue injury tolerances and relevant mechanical thresholds.

The anatomy and biomechanics of acute and chronic whiplash injury

Whiplash injury is the most common motor vehicle injury, yet it is also one of the most poorly understood. Here we examine the evidence supporting an organic basis for acute and chronic whiplash injuries and review the anatomical sites within the neck that are potentially injured during these collisions. For each proposed anatomical site--facet joints, spinal ligaments, intervertebral discs, vertebral arteries, dorsal root ganglia, and neck muscles--we present the clinical evidence supporting that injury site, its relevant anatomy, the mechanism of and tolerance to injury, and the future research needed to determine whether that site is responsible for some whiplash injuries. This article serves as a snapshot of the current state of whiplash biomechanics research and provides a roadmap for future research to better understand and ultimately prevent whiplash injuries.